Corrosion surveillance system

ABSTRACT

AN OL-ONE CORROSION SURVEILLANCE SYSTEM FOR MONITORING CORROSION DMAGES TO A STRUCTURAL COMPONENT CONTAINING A CORROSIVE LIQUID WHEREIN A FINE CONTROL ELEMENT IS, DURING OPERATION, DRAWN LONGITUDINALLY THROUGH THE STRUCTURAL COMPONENT AND SUBSEQUENTLY MEASURED FOR CORROSION DAMAGE DUE TO ITS EXPOSURE TO THE CORROSIVE LIQUID WITHIN THE COMPONENT. THE CONTROL ELEMENT IS MADE OF THE SAME MATERIAL IS THE STRUCTURAL COMPONENT AND SPECIALLY CONSTRUCTED SEALING MEANS IS PROVIDED FOR PREVENTING ESCAPE OF THE CORROSIVE LIQUID FROM THE COMPONENT AS THE CONTROL ELEMENT IS MOVED THERETHROUGH.

United States Patent 3,734,690 CORROSION SURVEILLANCE SYSTEM Morris Kolodney, River Edge, N.J., and William Arbiter, Yonkers, N.Y., granted to the United States Atomic Energy Commission under the provisions of 42 U.S.C.

Filed Apr. 28, 1970, Ser. No. 32,542 Int. Cl. G01n 17/00 US. Cl. 23--230 C 21 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION In systems using potentially corrosive liquids, as for example, chemical coolants and chemical milling solutions, it is important to provide the system with the capability of monitoring damage or physical change to structural elements of the system which are exposed to the corrosive liquid during operation. One such application where this capability is particularly useful relates to nuclear reactors of the type utilizing liquid sodium as a cooling medium and, in particular, to sodium cooled reactors which are intended for breeding of nuclear fuel.

In its pure state, liquid sodium is a relatively harmless fluid even at temperatures of about 1200 F. However, certain impurities greatly increase the corrosive action of the liquid sodium on the coolant pipes through which the liquid sodium is distributed throughout the reactor. For instance, a high carbon content has a destructive eifect on certain steel alloys containing the refractory metals such as columbium, vanadium, or zirconium. High carbon content also is apt to cause rapid embrittlement of austenitic stainless steel. On the other hand an excessively low carbon content may be harmful to low alloy steels of the chromium-molybdenum type which may be decarburized and, consequently, weakened. Similarly, excessive oxygen content in liquid sodium causes a wasting corrosion of stainless steels which, due to the high oxygen content, are dissolved in the hotter regions of the system and redeposited in solid form in the cooler regions. It will be recognized that the redeposited material is apt to cause clogging and consequent stoppage of the coolant fiow through the reactor. Oxygen is also harmful to any refractory alloys that may be present.

In the past, conventional chemical analysis has been employed to detect and measure oxygen, carbon and other impurity concentration in a sample of the liquid sodium coolant. However, reliable sampling is ditficult to obtain and present analysis techniques are relatively slow. Due to possible change in the impurity content during the time needed to complete the analysis of the sample, it is difiicult to relate the results of the analysis to the then existing impurity content of the actual coolant. With chemical analysis techniques a particular impurity content is measured an an indication of actual damage caused by the measured impurity must be arrived at by inference and speculation. This is due, in part, to the fact that damage done to structural components of the reactor by impuriice ties in the sodium coolant varies with the chemical form in which the impurities exist in the sodium coolant.

In an attempt to solve the problems inherent in chemical analysis techniques, on-line instruments have been devised to measure the chemical activities of oxygen and carbon in the liquid coolant. While far more satisfactory than chemical analysis, these instruments are capable of measuring only the individual effect of carbon and oxygen activity and not the combined effect of these impurities. In addition, on-line instruments presently available must be used at carefully controlled temperatures different from the system temperature and thus a true environmental test reflecting the actual operative condition of the coolant at a given time is not obtained.

Other known systems utilize a control wire made of the same material as the structure for which corrosion data is required. In these systems the control wire is fed through a sample of the corrosive liquid and thereafter a determination of change due to corrosion is made by comparing a portion of the control wire exposed to the sample liquid with portion of the wire not yet exposed to the corrosive liquid. These systems are complex and costly since they generally require elaborate apparatus for withdrawing a sample of the corrosive liquid. Also, it is diflicult to maintain the sample at the same environmental conditions as the actual liquid from which the same is taken.

SUMMARY OF THE INVENTION In accordance with the teachings of the present invention an on-line corrosion surveillance system and method is provided for continuously monitoring corrosion damage to structural components of a nuclear reactor such as coolant pipes which are exposed to potentially corrosive liquids such as a liquid sodium coolant flowing through the coolant pipes during operation of the reactor; and, moreover, one which provides a true environmental indication of actual corrosion damage without reference to the particular impurity content in the liquid sodium coolant causing the corrosion damage.

In construction, the system includes a fine elongated control element made of the same material as the coolant pipe. The control element may have a solid or hollow construction. The control element is moved longitudinally along a path leading diametrically through the coolant pipe and then through conventional reasuring instrumentation capable of measuring a corrosion indicative property of the control element as for example, its resistance, its permeability, its thickness, etc. The output signal from the measuring instrument is displayed on a conventional readout indicator.

Sealing means is provided to prevent escape of the liquid sodium coolant from the coolant pipe as the control element is drawn therethrough. This means comprises two cylindrical sealing chambers disposed in communicating relationship with the interior of the coolant pipe at the point of entry and exit of the control element therethrough. The size of cylindrical chambers is quite small having a diameter in the range A" to l and a length in the range of /2 to 1". With this arrangement, the normally hot liquid sodium standing in each sealing chamber is cooled to the vicinity of its freezing point. External cooling fins may be provided for each cylindrical chamber to aid cooling of the liquid sodium.

In one embodiment of the present invention the sodium coolant remains in a relatively cold, but liquid, state and the inlet and outlet seals are compression seals disposed in the outer end of each cylindrical sealing chamber. This type of seal permits longitudinal movement of the control element but prevents extrusion of the liquid coolant out of the sealing chamber.

In another embodiment, the inlet and outlet seals are formed by a column of solid sodium disposed within each sealing chamber. The solid sodium column is formed by allowing the liquid sodium standing in each sealing chamber to cool to at least its solidification temperature to thereby form a solid sodium sealant of at least /2 inch in length within each sealing chamber. With this arrangement the control element moves longitudinally through a small annulus extending in axial alignment through each of the solid sodium columns. The annulus through which the control element moves is sufficiently small in relation to the length of the solid sodium column so that substantially no liquid sodium flowing within the coolant pipe is extruded through the annulus during movement of the control element.

With the above described system a true environmental indication of corrosion damage to structural components of the reactor is obtained. This is achieved without reference to a particular impurity content in the liquid sodium coolant and without need to introduce complex and often unreliable sampling procedures commonly found in prior art systems. The system operates continuously in conjunction with the operation of the reactor itself thus permitting the status of the reactors structural components to be constantly monitored during all critical operating periods. In addition, the components of the system are simple, inexpensive, and easy to install. Also, the use of a solid sodium column for the purpose of sealing the coolant material itself forms an adequate seal thereby rendering the system self-sealing.

DETAILED DESCRIPTION OF THE INVENTION As shown in FIG. 1, the corrosion surveillance system of the present invention includes an elongated corrosion control element, indicated generally by reference numeral 10, which extends through nuclear reactor component 4 containing corrosive liquid 5 and testing means disposed in operative relationship to the control element exterior of the component 4 for measuring corrosion indicative mechanical and/or electrical properties of the control element such as strength, ductility, ability to resist bending, resistivity, magnetic permeability etc.

The output signal from the testing means is fed into a conventional readout indicator which may be equipped with a suitable alarm system to alert the operator to corrosion damage in excess of a preselected control value. The testing means and readout indicator are indicated diagrammatically in FIG. 1 by reference numerals 11 and 12 respectively. This instrumentation is sufficiently Well known to persons skilled in the art as to require no further explanation herein.

The reactor component 4 depicted in the drawings consists of a conventional coolant pipe of a nuclear reactor and the corrosive liquid 5 comprises liquid sodium coolant which is maintained at a temperature generally in excess of 900 Fahrenheit as it flows through the pipe 4. The reactor coolant pipe 4 is made of suitable material as, for example, austenitic stainless steel or, alternatively, of low alloy ferritic steel of the chromium-molybdenum type.

The control element is directed longitudinally from supply spool 6 along a path leading diametrically through the coolant pipe 4 around guide pulley 7 to the testing means 11 and then onto take-up spool 8. Drive means, indicated generally by reference numeral 13, is operatively connected to the take-up spool 8 to provide movement to the control element along this path. It is to be noted that the control element is drawn at a force not exceeding its yield strength either very slowly in a continuous manner through the coolant pipe or intermittently so that it is exposed to the hot liquid sodium for selected periods of time.

In the construction shown in the drawings a single testing means is located along the path of movement of the control element at a point located downstream of the point of exit of the control element through the pipe 4. It is to be recognized however, that a second testing means similar to the one previously described may be positioned along the path of movement of the control element at a point located upstream of the point of entry of the control wire through the pipe 4. With two testing means positioned in this manner, comparative measurements of corrosion indicative properties of the control element both before and after exposure to the liquid sodium coolant may be obtained. Also the control element may advantageously be subjected to a cleaning operation as by running it through an alcohol bath just after it emerges from the coolant pipe, as for example at 14, to remove any appreciable coolant film which might interfere with the measurements made by the testing means.

The control element is made of the same material as the coolant pipe so that a measurement of corrosion damage to the control element presents an accurate reflection of actual corrosion damage to the coolant pipe. The control element shown in FIG. 2 comprises a suitable length of solid wire 16 while the control element shown in FIG. 3 comprises a hollow construction comprising a suitable length the thin walled tubing 17. It is to be understood that the solid wire or the thin walled tubing may be used interchangeably as the control element in the corrosion surveillance system of this invention. In order to provide the system with suflicient sensitivity, the control element, whether of solid or hollow construction, is extremely fine in cros-sectional size. The control element is fine in the sense that it has a small enough diameter so that minute penetrations of its surface due to exposure to the corrosive liquid coolant are large enough to effect an appreciable reduction in its metallic crosssectional area. This construction provides the control element with a high degree of sensitivity as the percentage change in its corrosion indicative properties is appreciable.

When the solid wire is used as the control element a diameter of about .003 to .010 inch is acceptable and when the thin walled tubing is used as the control element, an outside diameter of about .003 to .015 inch with a wall thickness of .0005 to 0.002, respectively, is acceptable. Solid wire and thin walled tubing in these diameter ranges are commercially available. In the presently prefered em bodiment of this invention the solid control element has a diameter of .005 inch and the hollow control element has an outside diameter of .010 inch with a wall thickness of .002 inch. With this arrangement linear penetration of say .0001 and .0003 inch affects 8 and 23 percent, respectively, of the metallic cross-sectional area of the solid control element and 6 and 18 percent, respectively, of the metallic cross-sectional area of the hollow control wire. Disturbances of this magnitude are readily detectable yet are small enough to provide warning of corrosion damage to the coolant pipe in ample time for corrective measures to be initiated.

Escape of the hot liquid sodium from the coolant pipe is prevented by specially constructed sealing means. As shown in FIG. 1, this means includes two cylindrical sealing chambers 18 and 19 which are disposed on the exterior surface of the coolant pipe 4 at the point of entry and exit of the control element through the pipe. The sealing chambers 18 and 19 communicate with the interior of the coolant pipe via inlet and outlet holes 20 and 21, respectively, and the sealing chambers may be installed by welding them in place over their respecitve inlet and outlet holes. For purposes to be described more fully below, each sealing chamber is carefully sized to establish a relatively sharp temperature gradient across the liquidliquid interface between the hot liquid sodium flowing through the coolant pipe and the liquid sodium standing in each chamber.

In the construction shown in FIG. 2 the sealing means includes a compression seal disposed in the outer end of each sealing chamber. The compression seal, as shown,

is disposed in fluid tight relationship with respect to the inner surface of the sealing chamber and about in control element. In the presently preferred embodiment of this invention the compression seal is of the type using an elastomer bearing gland 25 which permits longitudinal movement of the control element therethrough while insuring that no liquid sodium is extruded through the interface between the control element and the gland.

With the construction shown in FIG. 3 the inlet and outlet seals are formed by a column of solidified sodium 22 standing within each of the cylindrical sealing chambers. It is preferred that the solid sodium column be at least /2" in length. The control element passes through a. small annulus 23 extending axially through the solid sodium column. The diameter of this annulus is slightly larger than the diameter of the control element but sufiiciently small in relation to the length of the sodium column to prevent liquid sodium flowing through the coolant pipe from being extruded through this annulus as the control element moves therethrough.

In order to form the solid sodium column each sealing chamber is sized so that the temperature gradient existing across the liquid interface between the sealing chambers and the coolant pipe is such that the liquid sodium standing within each sealing chamber is cooled to at least its freezing point of 200 Fahrenheit. In the presently preferred embodiment of this invention each sealing chamber is approximately /2 to 1 inch in length and approximately to 1 inch in diameter. With this arrangement a solid sodium column of about /2 inch in length is formed in the end sealing chamber. Cooling fins 24 extending radially outward of each sealing chamber may be advantageously provided to speed temperature reduction of the sodium coolant standing in each sealing chamber.

With the corrosion surveillance system of this invention several advantages, in addition to those previously discussed, are provided. First of all, the use of a fine control element as described above renders entry to and exit from the coolant pipe feasible without the necessity of constructing and installing complex and costly seals to prevent escape of the liquid sodium coolant from the coolant pipe during operation of the reactor.

The use of a hollow control element is of significant advantage for certain applications. It Will be recognized that the solid control element because of its small diameter is generally difiicult to handle and difiiculty is sometimes experienced in threading the solid wire forming the control element through the coolant pipe. Threading the solid control element is especially difficult with large coolant pipes having a diameter as large as 4 to 5 feet. However, a considerable increase in stiffness can be expected from a control element having the tubular construction described above. In addition, the outside diameter of a hollow control element of given metallic cross-sectional area can be made larger than the diameter of a solid control element having the same metallic cross-sectional area. Due to its increased diameter and due to its increased stiffness, the hollow control element is significantly easier to handle for installation purposes; and because the solid and hollow control elements have equal metallic cross-sectional area, the added advantages of the hollow control element can be obtained without sacrificing the sensitivity obtained with a solid control element. That is, regardless of the particular control element construction, the sensitivity of system is not disturbed.

We claim:

'1. A system for continuously monitoring corrosion damage to a structure due to exposure to a corrosive liquid contained within said structure comprising:

(a) a fine elongated control element;

(b) means for moving the control element in a lengthwise direction along a path having a first portion within said structure and a second portion without, such that said control element penetrates the structure and is exposed to said corrosive liquid during exposure of said structure;

(c) sealing means at each site of penetration of the structure for preventing escape of the corrosive liquid from the structure as the control element is moved therethrough; and

(d) testing means disposed externally of the structure along an outside portion of said path of movement for measuring at least one corrosion indicative property of the control element caused by its exposure to the corrosive liquid within said structure such that the corrosive damage of said structure may be determined by correlation with the corrosive progression of said control element.

2. The system according to claim 1 wherein a single testing means is disposed externally of the structure along an outside portion of the path of movement of said control element downstream of the portion of the path within said structure.

3. The system according to claim 1 wherein said outside portion of said path includes a first upstream portion whereby said control element enters the structure at a site of entry and a second downstream portion whereby said control element emerges from the structure at a site of exit.

4. The system according to claim 3 wherein said control element is made of the same material as said structure.

5. The system according to claim 4 wherein a first testing means is disposed along the first outside upstream portion of said path and a second testing means is disposed along the second outside downstream portion of said path such that comparative measurements of at least one corrosion indicative property of the control element before and after exposure to the liquid may be obtained.

6. The system according to claim 4 wherein said sealing means comprises:

(a) first and second sealing chambers disposed on the outer surface of said structure in communicating relationship'therewith at the sites of entry and exit, respectively, of the control element through said structure; and

(b) a compression seal disposed within the outer end of each sealing chamber in fluid tight relationship with respect to the inner surface of the sealing chamber and with respect to the control element extending therethrough.

7. The system according to claim 4 for monitoring corrosion damage to said structure due to exposure to a high temperature corrosive liquid flowing within said structure wherein:

(a) said sealing means comprises first and second sealing chambers disposed on the outer surface of said structure in communicating relationship therewith at the sites of entry and exit, respectively, of the control element through said structure; and

(b) each sealing chamber has a size such that a relatively sharp temperature gradient is established across the liquid interface between said structure and the sealing chamber such that the corrosive liquid standing within the sealing chamber is cooled at least to its solidification temperature thereby forming a seal of material within the sealing chamber.

8. The system according to claim 7 for monitoring corrosion damage to a coolant pipe of a nuclear reactor structure due to exposure to high temperature liquid sodium coolant flowing through said pipe wherein:

a) each sealing chamber is cylindrical in shape; and

'(b) the seal of solid material disposed within each sealing chamber comprises a column of solid sodium of at elast /2 inch in length.

9. The system according to claim 8 including:

(a) cooling fins extending radially outward of each sealing chamber.

10. The system according to claim 8 wherein:

(a) the control element has a hollow construction.

11. The system according to claim 10 wherein:

(a) the control element is constructed of thin walled tubing having an outside diameter of about .003 to .015 inch and a wall thickness of about .0005 to .002

1 inch.

12. The system according to claim 8 wherein the control element has a solid construction.

13. The system according to claim 12 wherein the control element is constructed of solid wire having a diameter of about .003 to .010 inch.

14. A method for monitoring corrosion damage to a structure due to exposure to a corrosive liquid contained within the structure comprising:

(a) moving a fine elongated control element in a lengthwise direction along a path having a first portion within the structure and a second portion without, such that said control element penetrates the structure and is exposed to said corrosive liquid during exposure of said structure;

(b) sealing each site of penetration of the structure by said control element for preventing escape of the corrosive liquid from the structure as the control element is moved therethrough;

(c) measuring at least one corrosion indicative property of the control element along an outside portion of said path; and

(d) correlating the corrosion progression of the control element to the corrosion progression of the structure thereby determining the corrosive damage to the structure.

15. The method according to claim 14 wherein outside portion of said path includes a first upstream portion whereby said control element enters the structure at a site of entry and a second downstream portion whereby said control element emerges from the structure at a site of exit.

16. The method according to claim 15 wherein said control element is made of the same material as the structure.

17. The method according to claim 16 for monitoring corrosion damage to a structure due to high temperature corrosive liquid contained within the structure wherein:

(a) the step of sealing the sites of entry and exit is accomplished by cooling the high temperature liquid around said sites to at least its solidification temperature to thereby from seals of solid material at said sites.

18. The method according to claim 17 for monitoring corrosion damage to a coolant pipe of a nuclear reactor due 0t high temperature liquid sodium coolant flowing through said pipe wherein:

(a) the step of cooling the high temperature corrosive liquid is accomplished by:

(1) forming a first and second sealing chamber communicating with the pipe on the exterior surface of the pipe at the sites of entry and exit of the control element through the pipe, said sealing chambers having a size such that a relatively sharp temperature gradient is established across the liquid interface between the sealing chamber and the pipe whereby liquid sodium standing within each sealing chamber is cooled to its solidification temperature forming a seal of solid sodium within each of said chambers.

19. The method according to claim 18 wherein:

(a) the sealing chambers are cylindrical in shape; and

(b) the solid soidum disposed within each chamber is at least /2 inch in length.

20. The method according to claim 18 wherein the control element has a hollow construction.

21. The method according to claim 18 wherein the control element has a solid construction.

References Cited UNITED STATES PATENTS 2,735,754 2/1956 Dravnieks 23230 3,013,569 12/1961 Sterczala 1341l3 'MORRIS O. WOLK, Primary Examiner R. M. REESE, Assistant Examiner US. Cl. X.R. 23-253 C 

